There are previously known apparatus and methods for improving the viewing angle of a "bi-level" LCD. An LCD is referred to as bi-level if it is switchable between a substantially light-transmissive state and a substantially opaque state, whereas a "gray scale" LCD has additional intermediate light transmissive states. Bi-level LCDs are commonly used in portable computer displays, whereas the poor viewing angle of gray scale LCDs has limited their use to small-screen devices such as portable television receivers. Before gray scale LCDs can be effectively used in large-screen television and color computer applications, viewing angle improvements are needed.
A clearer understanding of viewing angle factors follows with reference to FIG. 1. An LCD 10 having a front surface 12 and a rear surface 14 is illuminated from behind rear surface 14 by a light source 16. LCD 10 has a viewing axis 18 that is shown as a dashed line normal to the center of front surface 12 of LCD 10. An observer's eye located on viewing axis 18 at a position 20 views LCD 10 "on axis." If the observer's eye moves off axis to a position 22, LCD 10 is viewed at an off axis viewing angle 24 with respect to viewing axis 18. LCD 10 can be viewed with a constant viewing angle 24 anywhere around a viewing circle 26. Positions around viewing circle 26 are at an azimuthal angle 28 relative to a reference point 30 on viewing circle 26. As the observer views LCD 10 from various viewing and azimuthal angles, the perceived light may change in intensity, contrast ratio, or color. The degree of change may not be acceptable depending on the optical transmission characteristics of LCD 10 and the application.
FIGS. 2A, 2B, and 2C show, for a prior art twisted nematic LCD, light transmission percentage variation around viewing circle 26 for on-axis and 30 degree viewing angles and respective 0, 36, and 100 percent on-axis transmission levels. An ideal LCD would have light transmission percentage curves shown as concentric circles surrounding viewing axis 18 for all viewing angles. FIGS. 2A and 2C show respective near ideal light transmission curves 32 and 34 representing respective 0 and 100 percent (bi-level) transmission levels. However, FIG. 2B shows that for a 36 percent on-axis light transmission circle 36, light transmission at a 30 degree viewing angle degrades to an asymmetrical light transmission curve 38 varying from 2 to 93 percent transmission around viewing circle 26. Therefore, an intended 36 percent transmission level can be perceived by an observer as varying anywhere between "black" and "white," a clearly unacceptable variation.
FIG. 3 shows a prior art twisted nematic cell 40 of the type having the light transmission characteristics shown in FIGS. 2A, 2B, and 2C. The variation of off-axis light transmission represented in FIG. 2B is caused by asymmetry in a director field 41 of twisted nematic cell 40. Director field 41 is disposed between a pair of parallel electrodes 42A and 42B and includes elongated liquid crystal directors 43A through 43G (shown as lines), each oriented as indicated by the direction of the lines.
Director field symmetry or asymmetry is ascertained by assuming that twisted nematic cell 40 is bisected by an imaginary plane of symmetry 44 (shown in dashed lines) that is parallel to and equidistant from parallel electrodes 42A and 42B. For director field 41 to be symmetrical, directors disposed between electrode 42A and plane of symmetry 44 should be positioned and oriented with mirror-image symmetry relative to corresponding directors disposed between electrode 42B and plane of symmetry 44. Mirror-image symmetry exists if the half of director field 41 disposed between electrode 42B and plane of symmetry 44 can be inverted and superimposed on the half of director field 41 disposed between electrode 42A and plane of symmetry 44 such that the corresponding directors in both halves are spatially coincident. Directors 43A, 43D, and 43G lie substantially in respective planes 42A, 44, and 42B and are angularly twisted relative to each other. Clearly, mirror-image symmetry is not possible, and director field 41 is, therefore, asymmetrical.
Tunable birefringence type devices in which the directors are generally not twisted have also been used in LCD applications. Rather, the directors are generally co-planar with an imaginary cross-sectional plane oriented perpendicular to the cell electrodes.
FIG. 4 is an enlarged cross-sectional view of such a prior art tunable birefringence LCD cell 60 having a generally co-planar asymmetrical director field 62 disposed between a pair of transparent electrodes 64. A director in contact with a transparent electrode 64 is referred to as a surface contacting director 66. Any other director is referred to as a bulk director 68. The degree of birefringence, and therefore the light transmission, at any point in cell 60 is a function of the angle between a light ray and adjacent directors. Minimum birefringence occurs when the light ray propagates parallel to adjacent directors, and maximum birefringence occurs when the light ray propagates perpendicular to adjacent directors. The effective birefringence and light transmission of cell 60 is, therefore, a function of the average angle a light ray makes relative to all directors passed while traversing cell 60.
For example, electric potentials are applied to transparent electrodes 64 to hold director field 62 in an electric field of sufficient strength to provide an intermediate light transmission level through cell 60. A light ray 70 traversing cell 60 at an angle 72 has a minimum effective birefringence because the propagation direction of light ray 70 is somewhat parallel to a majority of directors 66 and 68. However, a light ray 74 traversing cell 60 at opposite angle 72 has a greater effective birefringence because the propagation direction of light ray 74 is somewhat nonparallel to a majority of directors 66 and 68.
The light transmissivity T (ranging from 0.0 to 1.0) of a tunable birefringence cell positioned between a pair of crossed linear polarizers is calculated from the equation: EQU T=sin.sup.2 (.pi..DELTA.n'd/.lambda.),
where the imaginary plane containing the co-planar directors is at a 45 degree angle relative to the polarization axes of the linear polarizers, .lambda. is the wavelength of the transmitted light, d is a distance 76 between electrodes 64, and .DELTA.n' is the effective birefringence of the cell.
FIG. 5 shows, for tunable birefringence cell 60 of FIG. 4, a light transmission variation curve 78 around viewing circle 26 for a 30 degree viewing angle and a 50 percent on-axis transmission level. The effective birefringence variation in cell 60 as a function of viewing and azimuthal angle (which are also the light ray propagation angles) causes the unacceptable variations in cell 60 light transmission.
Referring again to FIG. 1, a viewing angle of 30 degrees is not unusual. For example, if LCD 10 has a diagonal dimension 48 of 53 centimeters (21 inches), the eye of the observer at position 20, a distance 50 of 46 centimeters (18 inches) from LCD 10, views a corner 52 of LCD 10 at an angle 54 of about 30 degrees. Therefore, under optimum computer viewing conditions it is not possible to view an entire 21-inch LCD without some areas of the display being at a 30 degree viewing angle. 0f course, it is unlikely that the observer will always be centered on viewing axis 18 of LCD 10, and the viewing angle is even greater for certain individuals in a group of observers. This condition has hindered the use of color and gray scale LCDs in large-screen computer and television applications. Because the use of color and gray scale LCDs has been limited to small hand-held devices, many observers have not seen the gray scale viewing angle problem.
A "pixel" is the smallest addressable light transmission cell on an LCD, with a typical LCD having thousands of addressable pixels. Each pixel individually exhibits the same viewing angle characteristics exhibited by the entire LCD. In an LCD having multiple addressable pixels, each pixel can be thought of as an individual LCD cell. However, some LCD applications use a single large cell to cover an entire display surface. Therefore, LCDs can have as few as one cell or as many as thousands of cells. This invention applies equally to all such LCDs.
Prior workers have attempted to solve the gray scale LCD viewing angle problem by using multi-domain pixels in which the directors in one domain of the pixel are rotated 180 degrees from the directors in the other domain. The effective birefringence undergone by angled light rays is thereby averaged to improve the viewing angle characteristics of such an LCD. "Two-Domain Twisted Nematic and Tilted Homeotropic Liquid Crystal Displays for Active Matrix Applications, " K. H. Yang, Proceedings of the International Display Research Conference, IEEE, August 1991, pp. 68-72, describes a twisted nematic bi-domain pixel display with improved viewing angle characteristics in the horizontal viewing axis.
Other approaches to aligning directors with light rays are described in "Vertically Aligned Liquid-Crystal Displays," J. C. Clerc, SID Digest, 1991, pp. 758-761 ("Clerc"), and "Multi-Domain Homeotropic LCDs with Symmetrical Angular Optical Performances," H. L. Ong, SID Digest, 1992, pp. 405-408 ("Ong"). Both papers describe vertically aligned nematic ("VAN") cells (homeotropically aligned liquid crystal directors that have negative dielectric anisotropy) to improve viewing angle characteristics. Such cells have their surface contacting directors aligned substantially parallel to the viewing axis of the cell. However, the director fields in such cells are asymmetrical, and Clerc and Ong state that multi-domain pixels are required to achieve an acceptable viewing angle for such cells. Moreover, multi-domain pixel LCDs are generally costly, are difficult to manufacture, and have a light scattering wall between the pixel domains that reduces the contrast ratio of the cell.
Ong also describes the use of a negative birefringence compensating plate to improve hi-domain VAN LCD viewing angle characteristics. French Pat. No. 2 595 156 of Clerc et al. for "A Negative Birefringence Controlling Plate for use with Liquid Crystal Cells," published Sep. 4, 1987, describes methods for making such a plate. However, Ong shows that using a negative birefringence compensating plate does not dramatically improve LCD viewing angle characteristics and reveals a viewability compromise.
FIGS. 6A and 6B (reproduced from Ong) show optical transmission respectively with and without a negative birefringence compensator plate, graphed as a function of horizontal-axis viewing angle for each of four gray transmission levels of the bi-domain VAN LCD. For the uncompensated cell of FIG. 6A, the 4.3 volt transmission curve varies from 35 to 46 percent (a 31 percent change) over a viewing angle variation of 0 to 30 degrees, whereas in the compensated cell of FIG. 6B, the 4.3 volt transmission curve varies from 66 to 49 percent (a 25 percent change). However, for the uncompensated cell of FIG. 6A, the 10 volt transmission curve varies from 95 to 90 percent (a 5 percent change) over a viewing angle variation of 0 to 30 degrees, whereas in the compensated cell of FIG. 6B, the 10 volt transmission curve varies from 95 to 80 percent (a 15 percent change). The viewing angle is, therefore, improved by the negative birefringence compensator plate for transmission levels below 50 percent but is compromised for transmission levels above 50 percent.
Another type of prior art tunable birefringence cell is described in "The pi-Cell: A Fast Liquid-Crystal Optical Switching Device," by P. J. Bos and K. R. Koehler/Beran, Molecular Crystals and Liquid Crystals, 1984, Vol. 113, pp. 329-339 ("Bos and Koehler/Beran"), which describes the use of a symmetrical director field for achieving a wide viewing angle for a fast switching bi-level LCD. FIG. 7 shows an enlarged cross-sectional view of such a symmetrical LCD cell 80 having optically "self-compensating" director fields 82 and 82' symmetrically disposed about a cell centerline 83 located midway between a parallel pair of transparent electrodes 84 and 84'. Director fields 82 and 82' include respective surface contacting directors 86 and 86' as well as respective bulk directors 88 and 88'.
FIGS. 8A, 8B, and 8C show, for symmetrical LCD cell 80, light transmission percentage variation around viewing circle 26 for on-axis and 30 degree viewing angles and respective 0, 50, and 100 percent on-axis transmission levels. FIGS. 8A and 8B show acceptable bi-level transmission variations around respective 0 and 100 percent viewing circle curves 90 and 92. FIG. 8B shows that a 50 percent viewing circle curve 94 is more symmetrical than 36 percent viewing circle curve 38 (FIG. 2B) of the prior art twisted nematic cell. However, the viewing angle transmission variation of 20 to 82 percent is still unacceptable. In this regard the transmission viewing circle curves are similar to twisted nematic and many other cell types which have acceptable bi-level transmission viewing angles but have unacceptable viewing angles for intermediate transmission levels.
What is needed, therefore, is a low-cost, easily manufactured, color and gray scale LCD having wide and uniform viewing angle characteristic and a high contrast ratio. Such an LCD would be useful in large-screen color television displays.